I would consider this to be a personal/per site type of issue, for example certain types of sites lend themselves to ‘requiring’ registration, while others it’s more of an ‘un-natural’ or ‘awkward’ kind of thing. so, depending on what specifically you’re trying to do with your site, it’s something you should consider independantly of what ‘most’ others do, especially since there’s typically a ‘why’ attached to what folks are out there doing, and the why’s are very different from case to case.
i would say from a ‘spam’ perspective, as long as you’re using something like the akismet spam plugin, don’t worry about requiring registration for spam reasons.
I had required reg turned on when I first went up but I am keeping it among friends on my blog. I don’t advertise to google and I’m not trying to make money. Since one guy said he didn’t comment once because he didn’t feel like registering, I turned it off. I left a stern but kind warning that the first time I had to delete spam I would click it right back on.